Recycling products are used more and more for producing heat in industrial production methods. Particularly so-called secondary fuels are used for the production of cement and other ceramic products. These secondary fuels are used along with primary fuels such as coal dust and the like. The secondary fuels include dust type fuels, granulated fuels, pelletized fuels, flaked fuels and fiber secondary fuels, all of which are used in rotary kilns. These secondary fuels are produced of recycled garbage or waste materials which are shredded and sorted in accordance with material groups and then are delivered for use. The materials involved are for example, shredded plastic materials, paint dusts, carpet floor fibers, animal meal and other fuels made of garbage or of production scrap materials. In a few kilns operating in the cement industry already more than half of the fuel requirements are added as secondary fuels. These fuels gain more and more importance since the costs are small. The secondary fuels are generally supplied through dosing equipment into pneumatic transport conduits to be supplied into the combustion process. Cellular wheel sluices have proved themselves in practice for feeding into the pneumatic transport conduits. These sluices make possible a volumetric dosing under a pneumatic seal of the fuel supply.
German Patent Publication DE 200 06 800 U1 discloses such a fuel dosing with a cellular wheel sluice. A blow-through cellular wheel sluice is provided for the fuel dosing whereby the cellular wheel sluice axis extends in the direction of the main feed stream. A blower is arranged upstream of the cellular wheel sluice. The blower blows the secondary fuels out of the cellular wheel sluice into the transport conduit to the kiln. In connection with such blow-through cellular wheel sluices the problem occurs often that the blown-in air which is under a certain pressure of the air production reaches, as the cellular wheel keeps rotating, the point of material feeding as so-called scooping air where it exits directed opposite to the material flow which hinders the material flow. Simultaneously leakage air flows are generated through the gaps between the cellular wheel webs and the cellular wheel housing walls. These leakage air flows simultaneously blow back portions of the material through the gaps into the feeding chute. This makes a fuel dosing apparatus with such a blow-through cellular wheel sluice frequently ineffective and inaccurate because due to this reason it may come to a pulsating charging and thus to variations in the dosing precision. In this connection it also frequently happens that the dosing chambers are not completely emptied. In order to prevent this, more air is blown into the cellular wheel chamber whereby the reaction in the furnace is disturbed and may become ineffective.
For preventing such a scooping air and leaking air proportion, German Patent Publication DE 101 17 187 C1 discloses a cellular wheel sluice for secondary fuel dosing which is also constructed as a blow-through cellular wheel sluice. Thereby a cellular wheel having a relatively large inner core is provided by which the trapeze-shaped dosing chambers are formed only in the outer area of the radially extending webs. These dosing chambers are connected with an inlet pipe and an outlet pipe having a cross-section corresponding approximately to the size of the dosing chamber. Thereby, particularly the outlet opening is widened compared to the inlet opening, particularly for reducing the scooping air proportion so that the inlet opening is already closed as the cellular wheel keeps rotating while the outlet opening still has an opening gap in the widened areas. This feature is intended to cause a reduced pressure which sucks an air proportion out of the dosing chamber while the inlet opening is closed so that the scooping air proportion is reduced. Simultaneously, the cellular wheel sluice has sealing lips toward the housing at the ends of the dosing webs. These sealing lips are intended to close the leakage air gap so that a more continuous dosing and an improved chamber emptying is achievable by reducing the leakage air proportion. Such sealing lips which contact the housing are made as a rule of soft rubber-type sealing materials which seal the gap but nevertheless do not damage the housing walls nor do they cause wear and tear. However, secondary fuel materials frequently contain abrasive dust components, fiber remainders or metallic wire or nail remainders which damage the sealing lips making their functional life relatively short, thereby requiring frequent maintenance work.